Wirecut electrical discharge machines are known which employ a metal wire of approximately 0.05 to 0.3 mm in diameter as an electrode. The metal wire is fed in X and Y directions relative to a workpiece to perform machining operations, e.g., cutting and contour shape forming. The wire electrode is usually controlled to be relatively fed at a stepped, constant speed in units of 1 .mu.m per pulse, with the feedrate controlled so as to maintain a discharge of constant voltage in the machining gap without the need of controlling discharge energy, etc. However, when the thickness of the workpiece is not uniform, the workpiece is machined with the initial speed set at the speed corresponding to the maximum plate thickness (maximum area to be machined) in order to prevent short circuits between the wire and the workpiece or wire electrode breakage. In other words, the wire electrode is fed at the initially set low speed even though the plate thickness may have decreased during machining. Therefore, the overall machining efficiency is reduced.
A process for improving the above inefficiency was presented in Japanese Patent Publication No. 52890 of 1985. In this process, data combining the electrical conditions of a machining power supply suitable for various plate thicknesses of the workpiece and corresponding machining feedrates are stored in a memory. During operation, the data stored in memory is shifted to change the electrical conditions in correspondence with the machining feedrate so as to match the machining feedrate stored in memory with the machining feedrate during machining.
An example of this process will be explained in reference to FIG. 1, which illustrates a wirecut electrical discharge machine, wherein the numeral 1 indicates a wire electrode, 2 indicates a workpiece, 3 and 4 indicate upper and lower dielectric nozzles for injecting dielectric, respectively, 5 and 6 denote upper and lower wire guides for guiding the wire electrode 1, respectively, 7 indicates a feeder for feeding electrical power to the wire electrode 1, 8 designates a machining power supply, 9 denotes a table feed controller for controlling the movement of a table supporting the workpiece 2, 10 and 11 designate X-axis and Y-axis motors for driving the table in X and Y directions, respectively, and 12 indicates a numerical controller (NC) comprising a CPU, memories, a keyboard, a CRT, etc. The NC includes at least one memory which stores preset electrical conditions (ECs), i.e., peak machining currents I.sub.Pn, pulse widths .tau..sub.Pn, pulse-off periods .tau..sub.rn and capacitor capacities C.sub.n, and the upper and lower limits of relevant machining feedrates F which correspond to various plate thicknesses. The memory stores data as generally shown in FIG. 2, where, for example, if the plate thickness is in the range of 0 to t.sub.0, for a feedrate between FO and FO', the optimal electrical conditions are given by EC.sub.0.
FIG. 3 illustrates the changing of the electrical conditions in response to changes in the plate thickness of the workpiece 2. Assume that the workpiece 2 of plate thickness t satisfying the condition t.sub.3 &lt;t&lt;t.sub.4 is to be machined using electrical condition EC.sub.4. When machining is to be effected using this electrical condition, i.e., with a peak machining current of I.sub.P4, pulse width of .tau..sub.P4, width of .tau..sub.r4, and capacitor capacity of C.sub.4, the machining feedrate F is between F.sub.4 and F'.sub.4. Now assume that plate thickness t of the workpiece 2 changes to a thickness satisfying the condition t&lt;t&lt;t.sub.2, i.e., the plate thickness t decreases. Since the electrical condition was set to EC.sub.4, the actual machining feedrate F can be increased to exceed F.sub.4, the upper limit of the machining feedrate F of electrical condition EC.sub.4. Hence, a command is given to the machining power supply 8 to reduce the electrical conditions by one step, i.e., to EC.sub.3.
Since the actual plate thickness is smaller than the plate thickness of electrical condition EC.sub.3, the machining feedrate F can exceed F.sub.3 and the next electrical condition EC.sub.2 is then output. The electrical conditions are thus changed until the actual plate thickness matches the plate thickness of the electrical condition. In this way, this process automatically changes the electrical condition in accordance with the actual plate thickness, allowing more efficient machining of workpiece 2.
Electrical condition switching for the known wirecut electrical discharge machine designed as described above changes the electrical conditions in accordance with a change in plate thickness when the plate thickness varies as shown in FIG. 4. However, referring to FIG. 1, it will be noted that since the positions of the dielectric nozzles remain unchanged, spacings between the workpiece and dielectric nozzles change in accordance with the change in plate thickness. This, in turn, causes the pressures of the dielectric injected into the machining gap to increase when the spacings between the workpiece and dielectric nozzles are small and to decrease when the spacings are large. In addition, when the dielectric pressures are low, sludge and other deleterious materials produced by the electrical discharge occurring in the machining gap cannot be fully removed. It will be apparent that without proper sludge removal, the focused electrical discharge will break the wire electrode unless the electrical condition is changed.